Environmentally robust liquid crystal polymer coated optical fiber cable and its use in hermetic packaging

ABSTRACT

The invention relates to high-strength, abrasion-resistant optical fiber cable having a supplemental layer consisting essentially of a liquid crystal polymer (LCP) to enhance the cable&#39;s tensile strength and hermetically seal it, and an outermost encasing layer to protect the LCP supplemental layer from damage that could otherwise diminish the tensile strength or destroy the moisture barrier properties of the cable gained by adding the supplemental liquid crystal polymer layer. The encasing layer is preferably a thin layer of a smooth, non-crystalline thermoplastic that can be easily removed with chemicals that do not affect the properties of the supplemental layer so that the supplemental layer can be made accessible for promoting the formation of hermetically sealed interfaces between the cable and other structures. Cross-head extrusion methods for coating optical fibers with LCP and encasing layers are described along with laser and ultrasonic bonding techniques for fabricating hermetic packages.

CROSS REFERENCE TO RELATED APPLICATION

This application is continuation-in-part application claiming benefitfrom co-pending U.S. patent application Ser. No. 10/855,023 filed on May27, 2004 in the name of Amaresh Mahapatra, et al. and entitled “LiquidCrystal Polymer Clad Optical Fiber And Its Use In Hermetic Packaging.”

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of SBIR ContractNo. N00024-05-C-4102 awarded by the Naval Sea Systems Command.

FIELD OF THE INVENTION

This invention generally relates to fiber optical component packagingstructured to provide a hermetically sealed and moisture resistantbarrier that passes standard industry hermeticity and damp heatqualification tests. More particularly, it relates high-strength,abrasion resistant optical fiber cable that makes use of a supplementallayer of a liquid crystal polymer to enhance the tensile strength of thefiber cable, hermetically seal it, and promote the formation ofhermetically sealed interfaces between such coated fiber and otherstructures and an encasing layer to preserve the cable propertiesprovided by the LCP supplemental layer.

BACKGROUND OF THE INVENTION

Most electronic components, such as integrated circuits (ICs) forexample, are sealed within plastic packages. The plastic material issimply molded directly over the IC and a metal lead frame to which it isattached. However, this type of packaging is not particularly wellsuited for use with MEMS devices, where there is generally a need for anopen space within the package to accommodate motion of the mechanicaldevice within. In addition, effective packaging for MEMS and otherelectro-optical devices often needs to be comprised of hermeticallysealed housings to prevent the ingress of corrosive elements such aswater vapor and oxygen, isolate internal components from shock andvibration, shield the component from potentially harmful radiation, andprovide a means of conducting heat away from power dissipatingcomponents. In the case of electro-optic devices, the packaging mustalso provide a stable platform for the positioning and interconnectionof optical components, such as laser diodes, modulators, input andoutput fibers, and the like.

One of the most important features of a hermetic package is its abilityto withstand extended periods of “damp heat” and remain “dry” inside. Atypical hermetic test for telecom packages measures the package'sability to withstand 2000 hours in an environment of 85° C. at 85%relative humidity and remain “dry” inside; dry being defined as lessthan 5000 ppm internal moisture at the end of the test. Materialsconventionally used to achieve a hermetic seal are few: metal, glass,and ceramics. Packages sealed properly with these materials areconsidered truly hermetic. Common hermetic seal interfaces aremetal-to-metal seals, made via welding, brazing, or soldering;glass-to-metal seals; ceramic-to-metal seals; and glass-to-glass seals.

An example of a typical hermetic fiberoptic component package is a Kovarbox with a Kovar lid that is resistance welded in place via a seamsealer. Light passes in and out of the package via hermetic opticalpaths. Current methods of passing light through hermetic photonicpackages can be categorized as freespace or fiber feedthroughs.Freespace employs hermetic windows having metallized edges that aresoldered or brazed into the package wall, sometimes via an intermediatemetal ferrule or subcell. A hermetic collimator lens assembly issoldered to a metal package. Telecommunication grade optical fibertypically has a polymer cladding made of UV curable acrylate or Teflon.Hermetic seals cannot be made to these claddings since their moisturebarrier properties are inherently low. Hence, wherever optical fiberexits a hermetic package, the cladding layer must be stripped, and thebare silica fiber metallized. Afterwards, a hermetic seal is made to themetallization. Because bare silica fiber is fragile and often breaks,this process is inherently expensive. Hermetic fiber feedthroughs aremade using metallized glass fibers that are soldered to the package,typically via a cylindrical sleeve or support that protrudes from thepackage wall. The ferrule can then be hermetically attached to thepackage wall, typically soldered.

Fiber feedthrough ferrules with glass frits feature fibers sealed into ametal sleeve via a glass-to-metal seal using a glass frit between theglass fiber and metal sleeve (in this case the fiber is not metallizedbut its polymer cladding must be stripped). The ferrule can then besoldered to the package wall, or it may be part of the package wall.

Thus, there is a continuing requirement in the industry for low-costhermetic packages. Polymer packaging would be inherently low-cost;however, adhesives, epoxies, and polymers have not been shown to keepmoisture out of packages in extended damp heat tests. Some polymer-basedsealing methods and packages may satisfy limited test requirements, butmoisture diffuses through these materials over time. Nonhermetic andquasi-hermetic packages are suitable for certain applications. Thecomponent end-customer usually determines test requirements.

Recently, a new class of polymers, Liquid Crystal Polymers (LCP), hasbeen shown to have excellent moisture and oxygen barrier properties.Silicon Bandwidth, Inc., (Fremont, Calif.) and Foster Miller, Inc. (U.S.Pat. No. 6,320,257), have both proposed a liquid-crystal polymer packagethat can be metallized and soldered or welded to suitable lids toproduce packages that may not be strictly hermetic but may pass theTelecordia “damp heat” qualification test. Even with an LCP package, theproblems of producing an optical port remain; namely, stripping andmetallizing the fiber and soldering to the metallization. A need,therefore, exists for an improved technique to implement an optical portto be incorporated into metal, ceramic or LCP packages, and it is aprimary object of this invention to satisfy this need.

Additionally, it is known (See U.S. Pat. No. 4,778,244 to Ryan) toovercoat optical fiber with LCPs to provide an optical cable withenhanced strength which, in turn, may be over coated with scuffresistant coatings. However, the scuff resistant coatings described aredifficult to remove to allow ready access to the LCP coating for otherpurposes.

It is another object of this invention to provide optical fiber cablewith enhanced strength and easily removable encasing coatings to protectagainst environmental effects such as moisture and chafing forces.

It is yet another object of this invention to provide optical fiberstructures having properties for promoting the formation of hermeticseals when combined with other structures.

It is still another object of the present invention to providehermetically sealed packaging for optical and electro-opticalcomponents.

Yet another object of the present invention is to provide manufacturingprocesses for fabricating optical fibers coated with LCP and chemicallyremovable encasing layers.

Still another object of the present invention is to providemanufacturing processes by which hermetically sealed devices can befabricated with LCP materials and optical fibers having supplemental LCPand encasing layers to protect against environmental effects such asmoisture and mechanical forces.

Other objects of the invention will, in part, appear hereinafter andwill, in part, be obvious when the following detailed description isread in connection with the drawings.

SUMMARY OF THE INVENTION

The invention relates to high-strength, environmentally robust opticalfiber cables having a supplemental layer consisting essentially of aliquid crystal polymer (LCP) to enhance the cable's tensile strength andhermetically seal it and an outermost encasing layer to protect the LCPsupplemental layer from damage that could otherwise diminish the tensilestrength and moisture resistant properties of the cable gained by addingthe supplemental liquid crystal polymer layer. The encasing layer ispreferably a thin layer of a smooth, non-crystalline thermoplastic thatcan be easily removed with chemicals that will not affect the propertiesof the LCP supplemental layer so that the supplemental layer can be madeaccessible for promoting the formation of hermetically sealed interfacesbetween the cable and other structures. Crosshead extrusion methods forcoating optical fibers with LCP and chemically removable layers aredescribed along with laser and ultrasonic bonding techniques forfabricating hermetic packages.

In one aspect, the invention comprises an optical fiber cable having acore with a given index of refraction. The core is surrounded with acladding layer having an index of refraction lower than that of the coreso that the two in combination are capable of propagating light alongthe length of the fiber cable. At least one other layer surrounds thecladding and comprises a liquid crystal polymer material to enhance thestrength of the fiber cable, hermetically seal it, and promote theformation of hermetically sealed interfaces between the optical fibercable and other structures. The core and cladding are preferably of purefused silica with the core slightly doped with an index raisingmaterial. LCP coatings may also be applied to optical fiber cableshaving acrylate buffer layers over pure fused silica cladding and innercore. Surrounding the LCP supplemental layer is a thin chemicallyremovable encasing coating to maintain the strength enhancing, moisturebarrier, and interfacing benefits gained by the presence of the LCPlayer, but is otherwise easily removable under field conditions viachemicals that do not destroy the properties of the LCP layer,especially its ability to form hermetic seals with other structuresfabricated of LCP.

It is preferred to use a vertical drawing process with a crossheadextruder to apply the LCP and encasing layers over fibers formed of purefused silica claddings and inner cores. Use of a horizontal crossheadextrusion process may be made for applying the LCP and encasing layersover fibers that already have exterior polymer buffers such as acrylateor the like.

Packaging systems for hermetically sealing opto-electronic components,while providing ports for exchanging signals with components outside ofthe system, are also provided. Such systems comprise a housing forholding at least one opto-electronic component in place within thehousing and include means for providing a port for receiving and holdingat least one optical fiber cable adapted to optically connect with anopto-electronic component to provide a conduit for exchanging signalswith the optoelectronic component. The housing is composed of a materialfor at least in part hermetically sealing the opto-electronic componentwithin it. An optical fiber cable having a liquid crystal polymer layerand an exterior chemically removable encasing layer is provided toenhance the strength of said optical fiber cable, hermetically seal it,and promote the formation of a final hermetic seal at the housing portwhere the LCP layer of said optical fiber cable, upon the selectivelocal removal of the encasing layer, interfaces with the housing tocomplete the hermetic seal of the opto-electronic component within thehousing.

Housings comprise substrate bases, shallow casings or gaskets, and capsthat are provided with materials and features making them amenable tofabrication using laser bonding and ultrasonic welding to providehermetic ports and seals.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and methodology of the invention, togetherwith other objects and advantages thereof, may best be understood byreading the detailed description in connection with the drawings inwhich each part has an assigned a label and/or numeral that identifiesit wherever it appears throughout the various drawings and wherein:

FIG. 1 is a diagrammatic cross-sectional view of the structure of anoptical fiber cable in accordance with the invention;

FIG. 2 is a diagrammatic perspective view of a vertically orienteddrawing apparatus with a crosshead extruder for applying LCP andencasing layers over a fiber shortly after being drawn from a preform;

FIG. 3 is a diagrammatic illustration showing how LCP material becomesaligned as it passes through an extrusion head during coating;

FIG. 4A is a diagrammatic cross-sectional view of the structure ofanother optical fiber cable in accordance with the invention;

FIG. 4B is a flow chart of a process for applying the outer layers onthe optical fiber cable shown in FIG. 1

FIG. 5 is an exploded diagrammatic perspective view of components usedto hermetically seal a device in accordance with the invention;

FIG. 6 is a diagrammatic elevational view illustrating an arrangementoptionally employing a small ridge molded into an LCP cap to furtherincrease the localized pressure in forming a joint between an LCP capand a transparent substrate;

FIG. 7 is a diagrammatic perspective view of a hermetically sealedpackaging arrangement with an LCP cap and substrate along with a cutaway pressure plate and a scanning laser beam used for bonding purposesto form hermetic seals;

FIG. 8A is a graph of the absorption spectra of silicon illustratingthat it is essentially transmissive to radiation with wavelength aboveabout 1 μm;

FIG. 8B is a graph of the transmission of silica showing that it istransmissive to radiation in the wavelength range from 0.2 μm to 4 μm;

FIG. 9A is an exploded diagrammatic perspective view, with parts brokenaway, of a well-known hermetically packaged electro-optical device;

FIG. 9B is an exploded diagrammatic perspective view, with parts brokenaway, of an LCP packaged device in accordance with the invention;

FIG. 10 is a diagrammatic perspective view, with parts broken away, of asubstrate machined with a small dovetail shaped lip over which LCP ismolded, along with an enlarged view of a the detail of the dovetail;

FIG. 11 is a diagrammatic perspective view along with a detailedenlargement for a design for over molding electrical feedthroughs in LCPpackaging in accordance with the invention;

FIG. 12 is a diagrammatic perspective view, with parts broken away,showing an ultrasonic horn welding the components of an LCP package inaccordance with another aspect of the invention;

FIG. 13 is a diagrammatic elevational view showing specially configuredcomponents to be ultrasonically welded in an LCP arrangement usingenergy directors in accordance with another aspect of the invention; and

FIG. 14 is a diagrammatic perspective view, with parts broken away, ofan ultrasonic horn configured to form hermetic seals in accordance withthe invention, along with an enlarged view of an energy director.

DETAILED DESCRIPTION

The invention relates to high-strength, abrasion-resistant optical fibercable having a supplemental layer consisting essentially of a liquidcrystal polymer (LCP) to enhance the cable's tensile strength,hermetically seal it, and provide interfacing and an outermost encasinglayer to protect the supplemental layer from damage that could otherwisediminish the properties of the cable gained by adding the supplementalliquid crystal polymer layer. The encasing layer is preferably a thinlayer of a smooth, non-crystalline thermoplastic that can be easilyremoved with chemicals that will not affect the properties of thesupplemental layer so that the supplemental layer can be made accessiblefor promoting the formation of hermetically sealed interfaces betweenthe cable and other structures. Methods for hermetically sealingpackaging are also provided.

Reference is now made to FIG. 1 which shows the cross-sectional shape ofa high-strength, abrasion-resistant optical fiber cable 10 according tothe invention. As seen, optical fiber cable 10 comprises a core 12, acladding 14, a supplemental layer 16 composed of LCP, and a thin,chemically removable encasing layer 18. Core 12 and cladding 14 arepreferably formed of pure fused silica (SiO₂), but may be beneficiallycomposed of other suitable materials, as well. In addition, core 12 isslightly doped with an index raising material, such as Germania (GeO₂),so that its index of refraction is slightly higher than that of cladding14. Core 12 and cladding 14 collectively operate by total internalreflection to confine radiation in core 12 as it propagateslongitudinally along the fiber's length.

Generally, core 12 and cladding 14 are fabricated by drawing down apreform made from a well-known modified inside chemical vapor depositionprocess (MCVD) or OVD (outside vapor deposition) or VAD (vapor phaseaxial deposition). During the drawing process, LCP supplemental layer 16is applied directly to the fiber by means of a vertically orientedextruder. The liquid crystal polymer layer 16 is applied to enhance thestrength of optical fiber cable 10, hermetically seal it, andfacilitate, in a manner to be described later, the formation ofhermetically sealed interfaces between optical fiber cable 10 and otherstructures, as for example, hermetically sealed component packageddevices.

Extruded LCPs have high tensile strength, a measure of the force perunit cross-section at which an extruded tube of LCP will break. However,as we have discovered through extensive experiments, this does not meanthat LCPs are safe from external environmental effects such as moistureand mechanical forces such as chafing forces applied over time. Forexample, abrasion from chafing forces is a form of damage that resultswhen the surface of a material is rubbed against the surface of anothermaterial. In many potential applications, optical cables need to havehigh tensile strength so that they do not break as a result of thetension during deployment. They also may need to be abrasion resistantsince deployment may involve pulling through a conduit where the cablesurface will rub against the conduit walls and thus be exposed tochafing forces.

We have determined experimentally that extruded LCP layers on opticalfiber do not possess sufficient resistance to environmental effects suchas abrasion, and have developed an abrasion test method to determineabrasion resistance. A fine grit (120 grit) rubberized abrasive wheel isused for abrasion. A length of LCP coated optical cable is securelyfastened to a rigid horizontal surface. The test section is continuouslyabraded, in a reciprocating manner for 100 cycles, by a 25 mm squaresection of the rubberized abrasive element. The load on the abrasiveelement during test is a minimum of 500 gm. Following the test, thesample is examined under a microscope to determine the extent of surfacedamage. The sample passes the test if the breaking strength afterabrasion is no less than 90% of the breaking strength before abrasion.As a consequence of our testing, we have determined that it is necessaryto provide an encasing layer 18 over the LCP supplemental layer 16 topreserve the strengthened properties and hermetic seal of optical fibercable 10 which otherwise would be diminished considerably absent theencasing layer 18. Results have indicated that the enhanced strengthmade possible by the LCP layer can be reduced by abrasion by as much as41.7%, an unacceptably high level.

We have found that resistance to environmental effects can be madeacceptable by extruding a thin layer of a smooth, non-crystallinethermoplastic on top of the supplemental LCP layer. The polymer for thislayer is chosen not for its tensile strength, but for its ability toproduce a smooth, surface resistant to externally applied mechanicalforces tending to abrade or otherwise mechanically damage thesupplemental LCP layer.

In addition to the protective properties of layer 18, it must also beeasily removable under anticipated conditions of use in the field sothat it can be selectively removed to expose the underlying LCP layer sothat the supplemental LCP layer can be interfaced (bonded) with otherstructures to provide hermetically sealed systems. To this end, thecomposition of the encasing layer 18 preferably is chemically removableby exposure to softening agents that permit small, but controllable,lengths to be wiped away without damaging or changing the underlyingproperties of the supplemental LCP layer 16.

Suitable compositions of the encasing layer 18 include polyamides such aNylon 12 and polyesters. Nylon 12 may be rendered removable (strippable)by exposure to Chromic Acid (50%), Hydrochloric acid (37%), Nitric Acid(50%), or Sulphuric Acid (50%) while polyamides, in general, can besoftened with dichloromethane, combined with trifluoroacetic anhydride,which reacts with the amide groups and leads to a rapid dissolution ofpolyamides at ambient temperature. Benzyl alcohol may also be used at130° C. Warm benzyl alcohol presents few safety hazards. HFIP+potassiumtrifluoroacetate (mass fraction 0.2%). 1,1,1,3,3,3-Hexafluoropropan-2-ol(HFIP) is also a quasi-universal solvent for polyamides at ambienttemperature.

Polyesters may be rendered strippable by exposure to Methylene Chloride,acetone, methanol, and ethanol.

The material composition of encasing layer 18 should also be extrudable,preferably in the form of a thermoplastic.

Reference is now made to FIG. 2 which shows apparatus by which opticalfiber cable 10 is fabricated with supplemental LCP layer 16 and encasinglayer 18, both applied serially to cladding 14 that, in turn, surroundscore 12. As seen in FIG. 2, supplemental LCP layer 16 is applied tocladding 14 during the fiber drawing process, as it is essential thatthe fiber (core 12 and cladding 14) be protected from environmentaleffects immediately upon being drawn.

A fiber perform 11, from which core 12 and cladding 14 is drawn, is fedvertically into the top of a high temperature furnace 13, where the tipof perform 11 is heated until it softens. A thin glass fiber consistingof core 12 and cladding 14 is then drawn from the bottom of perform 11and fed through the remainder of the coating apparatus. To apply thesupplemental LCP layer 16 to cladding 14, an extruder 15 feeds moltenLCP polymer under pressure into a conical shaped tip 17 surrounding thefiber. The molten polymer thus forms a tight fitting sleeve around thefiber and is cooled and solidified by passing it through cold watercontained in a cylindrical water trough 19. The water in trough 19 iscirculated and cooled with fiber 10 passing through it over two pulleywheels, 21 and 23, respectively. Afterwards, the encasing layer 18 isapplied with apparatus (not shown) similar to that used to apply LCPsupplemental layer 16. Finished fiber cable 10 is finally wound onto alarge diameter fiber take-up storage drum 25. This entire process may beunder the supervision and control of a computer with appropriatesoftware to schedule and monitor the apparatus, associated activities ofthe apparatus, and communicating instructions and data among thecomponents of the apparatus.

Referring back to FIG. 1 again, core 12 is preferably single mode havinga diameter in the range between 5 to 8 micrometers, the diameter ofcladding 14 extends out to 125 micrometers, and the thickness ofsupplemental LCP layer 16 is 75 micrometers, and that of the encasinglayer 18 is about 40 micrometers or less, making the final diameter ofoptical fiber cable 10 about 440 micrometers. Supplemental LCP layer 16may be increased in thickness to increase the tensile strength ofoptical fiber 10.

Optical fibers are thin filaments of glass typically about 0.005″ indiameter. As described above, they consist of a central core regionsurrounded by an optical cladding, the refractive index of which isslightly lower than that of the core. Light launched into one end of thefiber is guided, essentially without loss, via total internal reflectionat the core-cladding interface. Optical fibers are inherently verystrong. Indeed, fused silica of which many fibers are made is one of thestrongest materials known, with a modulus of 10⁷ psi. During the hightemperature draw process, the surface of the optical fiber isessentially flame polished making it very smooth and clean. As the fiberis drawn from a furnace, a primary buffer coating of a relatively softelastomeric material is applied. In commercial optical fiber, thetypical primary coating is UV cured acrylates, or Teflon. This coatingserves to maintain the polished surface of the fiber and also to preventthe fiber from bending too sharply, which would cause stress and alsolead to increased optical attenuation. Typically, the primary coating isabout 60 μm thick, giving a final diameter of the primary coated fiberof about 250 μm.

To create a fiber cable, the optical fiber is further protected byadditional layers of extruded polymer and in many cases, longitudinalstrength members, made from materials such as Kevlar.

Apart from the advantages offered in terms of packaging, LCP coatingsalso present a means of creating very strong, compact and lightweightoptical fiber cables. For these applications, optical fiber structuresof the type shown in FIG. 4A may be used. Here, the optical fiber,designated at 50, has a slightly doped SiO₂ core 52 surrounded by anSiO₂ cladding 54, in turn, surrounded by an acrylate buffer 56.Surrounding the acrylate buffer 56 is a supplemental LCP layer 58, again75 or more micrometers in thickness. Beyond the supplemental LCP layer58 is an encasing layer 60, such as that previously described as layer18 in FIG. 1. Thus, the use of LCP as a supplemental layer or coating ontop of a conventional primary buffer or coating of commercial opticalfiber is shown. Such cables have applications in precision guidedmunitions, such as TOW missiles and “wire” guided torpedoes, local areaoptical communications (plenum cables), and optical fiber sensors. Thereappear to be no other materials that can be extruded onto optical fibersthat would provide the mechanical strength, thermal stability, andmoisture barrier properties of LCPs.

Reference is now made to FIGS. 4A and 4B, which show a method forapplying supplemental LCP layer 58 and encasing layer 60. After thebasic structure comprising core 52 and cladding 54 and a primary buffercoating 56 consisting typically of acrylate is formed, it is stored on afeed spool 20 that is mounted for rotation around a journal. Uncoatedfiber leaves feed spool 20 and is guided by a positioning idler 22 to awell-known crosshead extruder 24 that is used to raise LCP materialabove its melt temperature and deposit it as a layer on buffer coating56.

FIG. 3 illustrates in a general way how LCP material is organized andoriented as it passes thorough crosshead extruder 24, as will beexplained in more detail hereinafter. Afterwards, about 10 cm downstreamof crosshead extruder 24, the LCP coated fiber is sent through a waterbath 26 to solidify the supplemental LCP layer 58. Care must be taken tomake the water bath free of turbulence to reduce the possibility offorming a wavy exterior surface on supplemental LCP layer 58. Aftersolidifying, the diameter of supplemental LCP layer 58 is measured attest station 28 for conformity with its intended design specification.Then, another idler 30 guides the finished fiber structure onto atake-up spool 32 or alternatively, extruder 24, cooling bath 26 and teststation 28 are repeated in serial fashion to apply encasing layer 60.Throughout the coating process, take-up spool 32 places the fiber undertension to remove it from feed spool 20 and draw it through all of theother process stations prior to being rolled up for storage andsubsequent use. Process control of all of the parameters of the variouscomponents and materials used in the coating process are under thecontrol of a process controller 34, or suitably programmed computer,that passes signals and data among the various components via networkconnectors 36-48, or the like. General housekeeping chores are alsounder the control of process controller 34.

Having described fiber structures employing supplemental LCP and easilyremovable encasing layers and their method of fabrication, LCP materialsthemselves will now be discussed, particularly their barrier properties.

Not only are LCPs highly impervious to moisture, but also, as a resultof their tightly packed crystalline nature, their interstices allow verylittle absorption of moisture or other gasses. Consequently,out-gassing, which is a problem for many polymers, is reduced toinsignificant levels. Further, since LCP is merely heated to becomefluid, there is no need to use solvated LCP, which eliminates anothermajor source of outgassing. The moisture absorption and transmissionproperties of LCPs are compared with other polymer classes in Table 1below. TABLE 1 Comparison of barrier properties of different polymerclasses Moisture absorption (% @73° F., Moisture 50% relativetransmission Polymer humidity, rate (gm/m²/ class Specific polymer perday day/atm./mil Polyester PET (poly ethylene 0.06 28 terephthalate)(immersion) Fluorinated PVDF 0.5 5.3 polymer (Polyvinylidene fluoride,Dupont TTR10AH9) Polyamide Nylon 1.2 Liquid crystal Vectran 200P 0.020.17 polymer (made by Tecona) Zenite (made by 0.002 Dupont) 0.05 (6 mth.Immersion)

From the table, it can clearly be seen that LCPs offer very significantimprovement both in terms of moisture absorption and transmission.

Table 2 below shows typical film properties of LCP versus polyethyleneterephthalate (PET) films (Lusignea, R. W., 1997, “Liquid CrystalPolymers: New Barrier Materials for Packaging,” Packaging Technology,October, 1997). The water transmission rate through a 25 μm LCP film is0.17 gm/m²/day for an ambient pressure of 1 atmosphere.

A typical electronic package including cap and housing may be 10 mm×10mm×2 mm. Assuming the whole package is made of LCP, the total surfacearea is 280 mm². The total amount of moisture that would penetrate sucha package in a year, M, is given by:$M = {{\frac{0.17\quad{gm}}{m^{2} \cdot {day}} \times 365\quad{days} \times 280 \times 10^{- 6}m^{2}} \approx {0.017\quad{{gm}.}}}$

If the wall thickness of the housing and cap is 1 mm instead of 25 μm,the moisture level in the cavity after 1 year drops to insignificantlevels. TABLE 2 Typical LCP film properties Biaxially- oriented PET filmLCP Film Tensile strength (kPa) 172,000 240,000 Tensile modulus (106kPa) 5.2 12.4 Oxygen permeability 78 @ 25 μm 0.23 @ 25 μm (cc/m²-24hr.-atm.) Water vapor transmission rate 28 @ 25 μm 0.17 @ 25 μm(gm/m²-24 hr.-atm.) (per ASTM F-1249) Upper use temp. (° C.) 120 Over200 Density (gm/cc) 1.4 1.4 Tear resistance, 35 595 Initiation, kN/mTear resistance, 9 to 53 175 to 525 Propagation, kN/m

From Table 2, the oxygen transmission rate through a 25 μm LCP film is0.23 cc/m²-24 hr.-atm. The total amount of oxygen that would penetratesuch a package in a year, O, is given by:$O = {{\frac{0.23\quad{cc}}{m^{2} \cdot {day}} \times 365\quad{days} \times 280 \times 10^{- 6}m^{2}} \approx {0.02\quad{{cc}.}}}$

Given the volume of the cavity, this translates into a partial pressureof 0.1 atmospheres, which would also become negligible if the wallthickness of the housing and cap is 1 mm instead of 25 μm.

Therefore, the moisture and oxygen barrier properties of LCP are morethan adequate. Coupled with the moisture and oxygen barrier propertiesof LCP, the invention provides reliable and simple bonding techniquesfor providing hermetically sealed packaging arrangements as will bedescribed in more detail hereinafter.

Having described the barrier properties of LCPs, it is also important tounderstand their thermal, mechanical and electrical properties.

LCPs are thermoplastic so that there is an intermediate temperature suchthat the LCP is made fluid without a break down of the crystalstructure. They typically melt at about 280° C. and are thermally stableto 350° C. The coefficient of thermal expansion (CTE) is very low, andhighly anisotropic, being lowest in the direction of molecularalignment. The actual bulk value of the CTE can therefore be controlledto some extent by either controlling the degree of orientation, or bylaminating layers with orthogonal orientations (See U.S. Pat. No.5,529,741 incorporated herein by reference). This is a desirablefeature, since it means that the CTE can be matched to that of thesubstrate material, thus significantly reducing stress associated withthermal cycling.

LCPs also exhibit very little creep. This means that microscopicfeatures produced by molding, embossing or other such processes willretain their sharp edges and dimensional stability. Complex packagingdesigns, as those to be described, are therefore possible, in whichfinely detailed features can be defined to locate, align and secure thevarious optical and opto-electronic components.

In the electronic industry, the use of polymers with high temperaturestability is not new. For instance polyimide, which is stable to 350°C., has been used for fabrication of flexible circuits in electronicassemblies. But, it has a tendency to absorb moisture, which interfereswith high frequency performance. It has been found recently that liquidcrystal polymers (LCP) have significantly lower moisture absorption, andtransmission while being stable up to 350° C. and can, therefore, beused in high frequency flexible circuits.

LCPs have a low dielectric constant and loss factor from 1 kHz to 45GHz. For instance copper clad Biac LCP, sold by W. L. Gore for flexcircuit applications, has a dielectric constant of 3.0 and a losstangent of 0.003 from 3 to 45 GHz.

The beneficial properties of LCPs make their use as optical fibersupplemental layers attractive for a variety of reasons. Liquid CrystalPolymers (LCPs) are aromatic polyesters with rigid rod like molecularstructures. When heated and extruded, these long crystalline segmentstend to align themselves in the direction of flow, much like logs in ariver, see illustration in FIG. 3. However, extruded LCPs form longpolymer chains that are principally aligned along the direction ofextrusion as shown in FIG. 3. Such structures are built of individual,but parallel, interlacing strands of polymer. Each strand has very hightensile strength. When a tensile force is applied to the structure, itexhibits high strength since the intertwined strands do not break easilyand do not slip past each other. Rubbing of the structure surfaceagainst another surface, over time, however progressively tears theindividual strands apart. Where this has happened, the structure losestensile strength and moisture resistance since, in these sections, theseveral strands are no longer interlaced but become broken. To preventdegradation of tensile and moisture barrier properties over time andexposure to environmental forces, the extruded LCP needs to be encasedby another material which is largely amorphous so that there are noindividual strands that can separate one from the other, hence the needfor the encasing layer previously described.

It is the high rigidity of these long crystalline segments that give theLCP materials their high modulus and low permeability. As indicatedearlier, this invention advocates the use of Liquid Crystal Polymers asa supplemental layer or coating of optical fibers. LCP materials thathave been found useful for the purposes of the invention comprise, forexample, certain grades of the Vectra® LCP line marketed by Ticona,Summit, N.J. 07901 (See http://www.ticona.com). Vectra® liquid crystalpolymers (LCP) are highly crystalline, thermotropic (melt-orienting)thermoplastics that deliver exceptionally precise and stable dimensions,high temperature performance and chemical resistance in very thin-walledapplications.

The Vectra series of LCPs are primarily HNA-HBA copolymers where thefraction of HNA and HBA can be modified to alter LCP properties (see,for example, p. 375, “An Introduction to Polymer Physics,” David I.Bower, Cambridge University Press 2002). HNA and HBA are acronyms forhydrobenzoic acid and hydroxynaphthoic acid, respectively.

The use of LCP as an accessible coating material in an optical fibercable significantly reduces the problems associated with creatinghermetically sealed optical ports in packaged electro-opticalcomponents. It has been demonstrated that, under the correct conditionsof temperature, time, and pressure, a hermetic bond can be createdbetween glass and LCPs, and it has been found that the temperature ofthe substrate to which the LCP material is to be bonded should be higherthan the melt temperature of the LCP material and be applied under aslight positive pressure. The use of optical fiber cable having asupplemental LCP layer made accessible by removing an encasing coatingis illustrated in FIG. 5 in the packaging of an electro-optic device.

Referring now to FIG. 5, there is shown an electro-optic modulator 102packaged according to the invention, although the principals of theinvention apply to any such device. The packaging for electro-opticmodulator 102 is indicated generally at 100 and consists of molded LCPparts, which have features for locating and securing input and outputoptical fiber cables, 104 and 106, respectively. The two halves of thepackage, LCP cap 108 and gasket 110, are brought together to capture theinput and output fibers, 104 and 106, in small molded, generallyelongated semicircular, recesses, 112 and 114, respectively, provided inthe adjoining faces of LCP cap 108 and gasket 110. It will be understoodthat the encasing layer of the optical fiber cable will have beenremoved over an appropriate length to permit its underlying supplementalLCP layer to reside in recesses, 112 and 114. The two halves of package100 are then fused together by heating. A scanning laser beam,ultrasonic or RF induction heating, or other functionally similartechnique may perform the heating. In the case of laser bonding, the LCPmaterials need to be sufficiently thin to allow the laser radiation toreach joint areas or the overlapping seal areas can be melted to fusethe LCP materials to form seals, and this can be done scanning from theside where an interface exists or perpendicular to the joint as, forexample, from the cap side.

Because the fiber cable supplemental layer or coating is made from thesame material as the other packaging components (LCP), a perfect seal iscreated when the two materials are fused together. The package ishermetic since the LCP material has excellent moisture and oxygenbarrier properties. Materials for the package can be selected from thosealready identified above.

With respect to the use of laser bonding, it was first demonstrated onthermoplastics in the 1970's, but has only recently found a place inindustrial scale situations. The technique, suitable for joining bothsheet film and molded thermoplastics, uses a laser beam to melt plasticin a joint region. The laser generates an intense beam of radiation,which is focused onto the material to be joined. This excites a resonantfrequency in the molecule, resulting in heating of the surroundingmaterial. Three forms of conventional laser bonding exist; CO2, Nd:YAG,and diode laser bonding. Table 3 below gives comparative data on laserbonding with CO2, Nd:YAG and diode lasers. TABLE 3 Comparison ofcommercially available laser sources for plastics processing Laser TypeCO₂ Nd: YAG Diode Wavelength 10.6 1.06 0.8-1.0 (μm) Max. power 60,000   6,000     6,000 (W) Efficiency 10% 3%    30% Beam Reflection Fiber opticFiber optic Transmission off mirrors and mirrors and mirrors Minimumspot 0.2-0.7 diam. 0.1-0.5 diam. 0.5 × 0.5 size * (mm)

Laser radiation is absorbed by polymers in molecular vibration spectraof covalent bonds such as C—H, C—C, C—O, C═O. The vibration spectra of aparticular type of bond are characteristic of that bond. The location ofthese vibrational states is determined largely by the spring constant ofthe bond and the reduced mass of the two nuclei. Since the vibrationalenergy is inversely related to the reduced mass, bonds with low reducedmass (such as C—H) have high vibrational frequencies. For example, theC—H, —C═C—, and —C—O— bonds have fundamental vibrational resonance atwavelengths of 2, 3.3 and 10 μm, respectively. Since C—H is the bondwith the lowest reduced mass in polymers, absorption below 2 μm isminimal. In practice, because of vibrational overtones, most polymersare transmissive below 1 μm and highly absorptive above about 1.5 μm.CO₂ radiation (10.6 μm) is readily absorbed by most polymers in lessthan a mm. Nd:YAG (1.06 μm) will penetrate several mm into most polymerswhile diode lasers (0.8 μm nominal) will not be absorbed by polymersunless IR absorbing dyes such as carbon particles are added. Since thereis some risk in degrading the barrier properties of LCP or generatingcarbonization by adding dyes or carbon powder, diode laser bonding isnot an option.

For electronic and opto-electronic packaging, an embodiment of theinvention uses a laser as a means to bond an LCP cap to a substrate suchas silica, silicon or ceramic (See FIG. 5). The ideal laser wavelengthmust be transmitted by the substrate and strongly absorbed by the LCP,in a thickness of a fraction of a millimeter. This allows laser deliverythrough the substrate to the junction that needs to be bonded.Mechanical pressure is applied to the bond region to ensure a viablebond. The LCP is sufficiently thick, e.g. 1 mm, so that only theinterface layer melts, and thus, allows pressure to be applied to theunmelted layer of the LCP. To further increase the localized pressure, asmall ridge may optionally be molded into the LCP cap, as shown in FIG.6. The laser energy is absorbed within the raised ridge, causing it tomelt and bond with the substrate.

To bond an LCP cap to a substrate, the LCP ridge needs to be melted atall points. A scanning laser beam is employed (See FIG. 7) to accomplishthis. The laser wavelength must be such that it is absorbed strongly bypolymers—hence it must be >1.5 μm. Thus, Nd:YAG is not an option. Itmust also be transmitted by silicon, silica and ideally by ceramics suchas sapphire. FIG. 7A shows that silicon is essentially transmissiveabove about 1 μm. Silica, on the other hand, is transmissive in thewavelength range from 0.2 μm to 4 μm as shown in FIG. 7B. Alumina(sapphire) is transmissive from 0.15 μm to 6.5 μm. Hence, an idealwavelength for LCP bonding to all these substrates is in the range from1.5 μm to 4 μm.

An embodiment of the invention uses a thulium fiber laser (wavelength:1.8-2 μm) or Er:YAG laser (wavelength: 2.94 μm) for laser bonding LCP tosilicon, silica and sapphire. IPG Photonics in Oxford, Mass., hasdeveloped thulium fiber lasers, which operate in the wavelength range of1.8 to 2.0 μm range with a maximum output power of 100 W. These are CWlasers. CW lasers are typically better than pulsed lasers for polymerbonding since high peak power of pulsed lasers may result in localburning from impurities. Fiber lasers are ideally suited for beamdelivery in a laser bonding application. Erbium in YAG (Er:YAG) laserswhich operate at a wavelength of 2.94 μm and 30 W power have beendeveloped primarily for dermatology applications. They are manufacturedby companies such as Lynton Lasers in England and Unitech Corp in Japan.The wavelength is ideal for laser bonding LCP to silicon, silica orsapphire. Note that Er:YAG is not to be confused with Er in silicalasers and optical amplifiers (EDFA), which nominally operate at awavelength of 1.5 μm.

Having discussed the structure and fabrication of optical fiber cableshaving supplemental LCP and removable encasing layers and theirincorporation into a general form of hermetic package using laser and/orultrasonic bonding, more detailed embodiments of packaging will now bediscussed.

As indicated previously, the primary functions of packaging are:

To prevent the ingress of corrosive elements such as water vapor andoxygen.

To isolate the internal components from shock and vibration.

To shield the component from potentially harmful radiation.

To provide a means of conducting heat away from power dissipatingcomponents, and

In the case of electro-optic devices, the packaging must also provide astable platform for the positioning and interconnection of opticalcomponents, such as laser diodes, modulators, input and output fibers,and the like.

A typical, well-known, packaged electro-optic device is illustrated inFIG. 9A. This type of configuration, referred to as a Butterfly package,is used extensively in the packaging of electronic, electro-optic andmicro electro mechanical systems (MEMS). The casing of the package ismetal, and serves as both a barrier and a heat sink for the device,which is bonded to the base. Electrical connections to the device aremade via electrical feedthroughs, which are sealed using glass beads,which are bonded to both the casing and the electrical conductors. Suchglass-to-metal seals are used extensively in many types of packaging.

For an electro-optic device, such as the one illustrated, it isnecessary to couple optical signals in and out of the device via opticalcables. Creating a hermetically sealed optical port is a non trivialtask, and typically involves metallizing the optical fiber cable so thatit can soldered into a metallized glass bead which in turn is solderedinto a small hole in the metal casing. To metallize the fiber, theprotective outer buffer coating must first be removed, rendering thefiber very susceptible to moisture degradation and breakage. Creatingthe sealed optical port is therefore a very delicate, time consuming andexpensive operation, and is one of the major factors contributing to thevery high cost of packaging this type of component.

In accordance with a further aspect of the invention, the metal housing,glass beads and metalized fibers are replaced with a simplified,precision molded package, in which the necessary integrity is achievedby bonding the LCP directly to various components. In the case of theoptical port, use is made of the LCP coated optical fiber cable aspreviously described, which can be thermally bonded directly into ahousing after having had its encasing layer removed. Referring to FIG.9B, there is shown an inventive package designated generally at 200comprising a metal substrate 202 to which an actual device 204 isbonded. Substrate 202 serves primarily as a thermal heat sink for device204. For devices with low power dissipation, the metal substrate isalternatively replaced by LCP.

An LCP gasket 206 or shallow casing is bonded to the metal substrate202, with electrical feedthroughs 208 (typical) molded in. One or moreoptical ports are made using an LCP ferrule 210 that is molded to theoutside of an LCP supplemented fiber 212. The ferrule 210 is designed sothat it can be bonded to the LCP gasket 206 using ultrasonic bondingtechniques to be described later. Finally an LCP cap 214 is bonded on tothe top of the gasket 206, which may be accomplished using ultrasonicbonding.

A method of the invention for assembling an LCP housing can besummarized in five major steps:

Bonding an LCP gasket or shallow casing to a metal substrate oralternatively LCP substrate.

Producing hermetically sealed electrical feedthroughs.

Manufacturing supplemental LCP coated optical fiber cable with removableencasing layers.

Producing hermetically sealed optical feedthroughs.

Ultrasonically welding and bonding the LCP cap to the LCP gasket.

The first step in the packing process or method is to attach an LCPgasket to a metal or LCP substrate. LCP films are routinely bonded tothin copper laminates to produce flexible circuit boards, and thestrength of the bond between the LCP and the copper is extremely high.However, in this particular situation, both materials are very thin, andstresses due to thermal expansion mismatch are relatively low. When morerigid and substantial components are connected together, the problem ofthermally induced stresses becomes more significant, and it is probablethat repeated thermal cycling could result in a failure of the LCP/metalbond. For this reason, the LCP gasket 206 is preferably attached to themetal substrate 202 by “over-molding” using a mechanical keying featuremachined into the substrate 202. Over molding is a process whereby apolymer material is injection molded in a cavity into which the matingpart (in this case, the substrate) has been placed. Here, the substrateis machined to produce a small dovetail shaped lip 216, over which theLCP is molded, as shown in detail in FIG. 10. The shape of the lip 216ensures that, as the two materials expand and contract, the seal betweenthem remains tight.

As with the gasket to substrate seal, a major problem to overcome withthe electrical feedthroughs is that of preventing bond failure as aresult of stresses resulting from mismatched expansion coefficients. Aswith the LCP to substrate seal, this problem is overcome by molding theLCP directly over the electrical output pins, and designing mechanicalfeatures into the latter, such that the two components are essentiallylocked together.

An embodiment of the over-molded electrical feedthroughs is shown inFIG. 11. Electrical pins 220 are manufactured either by machining orstamping, and are mounted onto a lead frame for insertion into theinjection molding cavity. Ridges 222 (typical) are preferably providedon the metal conductors to further ensure a high integrity seal.Pedestals or ledge 224 are located underneath electrode pads 226 forlocation and support purposes.

Another aspect of the invention comprises extensive ultrasonic weldingto bond various components together. Ultrasonic bonding is a techniquewidely used in the manufacture of plastic components. The materials tobe welded must be thermoplastic. That is, they must not change theirchemical composition upon melting. The process relies upon the use ofhigh frequency sound waves to create localized heating by friction. Theheating softens the thermoplastic material and causes the two parts tobe fused together. In the bonding process, the two parts to be weldedare held together under pressure and are then subjected to ultrasonicvibrations usually at a frequency of 10 to 40 kHz.

The ability to weld a component successfully is governed by the designof the equipment, the mechanical properties of the material to be weldedand the design of the components. In order to ensure that heat isgenerated locally, at the joint between the two parts, the componentsare designed with small features called energy directors (See FIG. 13).These energy directors consist of small (typically 0.1″) ridges withpointed tops, which serve to focus ultrasonic energy into a relativelysmall volume of material, causing rapid heating and subsequent melting.

A typical ultrasonic bonding system is illustrated generally at 300 inFIG. 12. It consists of a transducer 302, booster 304, horn 306 (orsonotrode) and anvil 308. The two parts to be welded are sandwichedbetween the horn 306 and the anvil 308, which must be relativelyimmovable. Ultrasonic energy from the transducer 302 is amplified by thebooster 304 and coupled to the horn 306, which in turn transmits it tothe upper of the two components (Shown generally at 310).

The design of an embodiment of an LCP cap 400 in accordance with theinvention is shown in FIG. 14. An energy director 402 is provided in LCPcap 400, and an ultrasonic welding horn 404 is configured to operate inconjunction with the shape of the LCP cap 400.

While fundamental and novel features of the invention have been shownand described with respect to preferred embodiments, it will beunderstood that those skilled in the art may make various changes to thedescribed embodiments based on the teachings of the invention and suchchanges are intended to be within the scope of the invention as claimed.

1. An optical fiber cable comprising: an optical fiber having a corehaving a given index of refraction and a cladding layer surrounding saidcore and having an index of refraction lower than that of said core sothat the two in combination are capable of propagating light along thelength of said optical fiber; a supplemental layer surrounding saidoptical fiber, said supplemental layer consisting essentially of aliquid crystal polymer (LCP) material to enhance the tensile strength ofsaid optical fiber, hermetically seal it, and promote the formation ofhermetically sealed interfaces between said optical fiber cable andother structures; and an encasing layer surrounding said strengtheninglayer to protect it from damage that could otherwise diminish thetensile strength and hermetic seal of said optical fiber cable gained byadding said strengthening layer of said liquid crystal polymer material,said encasing layer comprising a thin, smooth, non-crystallinethermoplastic.
 2. The optical fiber cable of claim 1 further including abuffer layer, said buffer layer being located exterior to said claddinglayer and intermediate said first cladding layer and said supplementallayer.
 3. The optical fiber cable of claim 2 wherein said buffer layercomprises a polymer.
 4. The optical fiber cable of claim 3 wherein saidpolymer comprises acrylate.
 5. The optical fiber cable of claim 1wherein said core is composed of pure fused silica doped with an indexraising material and said cladding layer is composed of pure fusedsilica.
 6. The optical fiber cable of claim 1 wherein said core andfirst cladding layer are configured and arranged with respect to oneanother so that said optical fiber cable is single mode.
 7. The opticalfiber cable of claim 1 wherein said liquid crystal polymer material is athermotropic thermoplastic.
 8. The optical fiber cable of claim 1wherein said encasing layer is selected from the group comprisingpolyamides and polyesters.
 9. The optical fiber cable of claim 8 whereinsaid polyamide is Nylon
 12. 10. The optical fiber cable of claim 8wherein said polyamide is chemically removable with a softening agentselected from the group comprising dichloromethane, combined withtrifluoroacetic anhydride, benzyl alcohol at 130° C., and HFIP+potassiumtrifluoroacetate (mass fraction 0.2%). 1,1,1,3,3,3-Hexafluoropropan-2-ol(HFIP).
 11. The optical fiber cable of claim 8 wherein said polyestersare chemically removable with a softening agent selected from the groupcomprising methylene chloride, acetone, methanol, and ethanol
 12. Theoptical fiber cable of claim 1 wherein said encasing layer can be easilyremoved with chemicals that do not affect the properties of saidsupplemental LCP layer so that said supplemental layer can be madeaccessible to form hermetically sealed interfaces between said opticalfiber cable and other structures.